Optoelectronic devices

ABSTRACT

This invention relates to optoelectronic devices of improved efficiency. In particular it relates to light emitting diodes, photodiodes and photovoltaics. By careful design of periodic microstructures, e.g. gratings, associated with such devices more efficient light generation or detection is achieved.

This invention relates to improved optoelectronic devices, in particularit relates to optoelectronic devices such as light emitting diodes(LEDs), photodiodes and photovoltaic cells.

Optoelectronic devices include devices that generate light radiation andthose that detect light radiation. Typically such devices are used indisplays and sensors.

Many optoelectronic devices continue to be refined and improved. It isan on-going objective with regard to the development of such devicesthat they should be as efficient as possible. For example desirableimprovements include increased brightness from an LED or photodiodesthat may operate successfully with lower levels of input light.

An organic LED essentially consists of a light emitting layer sandwichedbetween an anode and a cathode. Typically the anode is in contact with asubstrate.

Electrons and holes combine in the light emitting layer to produce lightvia the decay of excitons. In addition to generating “useful” lightradiation, both heat and trapped optical modes may also be produced.

Usually light is emitted through the anode, however more recentlyorganic LED emission through a planar metal cathode has beendemonstrated by Hung et al, Appl. Phys. Lett., 78, 544 (2001).

The presence of a metallic cathode cannot usually be avoided. Inaddition to its necessary electrical function, it also significantlymodifies the optical properties of the structure. Such modifications canbe advantageous. For example where emission is to take place through a(semi-) transparent anode, light that was initially directed towards thecathode may be partially reflected so as to emerge through the anode andthus be recovered as useful radiation. However, metal surfaces also actto quench emission. Two distinct effects can act to quench emission froma light emitting layer if it is placed close to a metal electrode. Ifthe light emissive layer is very close to the metal layer, for exampleif the distance between the light emitting species and the metal is assmall as the emission wavelength/40, the energy of the emissive layercan be transferred directly to an electronic excitation of the metal,which results in conversion of the energy to heat. It is common practicein the fabrication of light emitting devices to design the structure insuch a way that the emissive zone in the light emitting layer is spacedaway from the metal electrode to minimise this effect. If the emissivespecies is at a greater distance from the metal electrode, for exampleat a distance of the emission wavelength/10, the energy can be capturedas a plasmon wave at the surface of the metal. On planar surfaces thesurface plasmon modes are non-radiative and thus act as a loss channelfor the device, so impeding efficiency, see Barnes IEEE J. Light. Tech.,17, 2170 (1999), and Hobson et. al. IEEE J. Sel. Top. Quant. 8, 378. Ina typical organic light emitting diode structure, if the emittingspecies is placed at a distance of 10 nm from a metal electrode, some60% of the energy will be directly lost to the metal. If the emittingspecies is placed at a distance of 50 nm from the metal electrode, theloss of energy to the metal is reduced to about 8%, but some 47% of theenergy is trapped as a surface plasmon mode.

In some applications emission through the anode may be eitherimpractical or undesired. One example is emission from quantum wellsembedded just below the surface of a high index semiconductor; hereemission through the semiconducting wafer may be impractical. Anotherexample is that of an organic LED built onto the surface of a siliconwafer. In both cases it would be convenient to accomplish emissionthrough a semitransparent metal electrode. It is one of the aims of thepresent invention to improve the efficiency of such a scheme.

It is well known that in fabricating a light emitting organic device, itis desirable to use an electrode material having a low electronic workfunction for the cathode. Such low work function materials aredesirable, because they minimise the energy barrier to the injection ofelectrons into typical organic semiconductor layers. Such low workfunction electrodes are normally metals such as calcium, lithium, therare earth or lanthanide metals and their alloys. It is also known thatcathode materials such as aluminium which have an intermediate value ofwork function together with an additional layer which promotes chargeinjection can be used. Such additional layers include a thin layer oflithium fluoride. It remains difficult to provide a low work functionelectrode which does not have a metallic character. Organic lightemitting devices in which emission cannot take place through the anodemust therefore, in general emit their light through a thin semitransparent metal cathode. The problem remains that much of the emittedlight is trapped as surface plasmons.

If nothing is done to recover trapped guided modes such as SP modes thenthey represent a wasteful decay route for the excitons that generate thelight and will reduce the external efficiency of the device.

Various attempts have been made to increase the light output from LEDs.Lupton et al Appl. Phys. Lett, 71, p 3340, 2000 discuss the use of adiffraction grating for increasing the brightness of the emitted light.

Typically such diffraction gratings are described as being added to thesurface of the optoelectronic device, however in reality what results isa periodic microstructure extending through a number of layers. Forexample FIG. 1 illustrates a typical LED and FIG. 1 a illustrates theLED of FIG. 1 after a periodic microstructure has been imparted on toit.

In FIG. 1, the LED comprises a glass substrate 1 on to which have beendeposited an anode 2, a light emitting layer 3 and a cathode 4 typicallymade from metal. The arrow indicates the usual direction of the emittedlight.

In FIG. 1 a the periodic microstructure is represented as a corrugatedlayer. Typically the glass substrate 1 is spin-coated with photoresist,baked and exposed to laser light such that a wave pattern is formed inthe photoresist. Following further processing and exposure to UVradiation to harden the photoresist, or use of reactive ion etching totransfer the pattern to the substrate, the anode 2, dielectric orsemiconductor layer 3 (often referred to as a light emitting layer) andcathode 4 are deposited. The effect of depositing further layers on tothe corrugated glass substrate is that this periodic microstructureextends through the subsequently deposited layers such that, in theexample illustrated by FIG. 1 a, the cathode 4 possesses a periodicmicrostructure.

In FIGS. 1 and 1 a the interface between, for example the cathode 4 and“above” would, in the absence of a further layer be commonly referred toas the cathode/air interface.

It is an objective of the current invention to provide more efficientoptoelectronic devices. This is realised partly by understanding morefully those processes occurring at the various interfaces ofoptoelectronic devices and the subsequent use of microstructures in sucha way that SP modes are recovered as useful radiation.

The above description of devices has been framed primarily in thecontext of devices which emit light. Such devices include light emittingdiodes, organic light emitting diodes, electroluminescent devices usingthin film or powdered phosphors and light emitting polymer devices.Those skilled in the art will immediately recognise that the essentialaspects of the discussion are common to these and to other lightemitting devices. It will also be recognised that the sameconsiderations apply to other electro-optic devices. For example, lightdetection and photovoltaic devices rely on light entering the devicethrough an electrode structure and causing excitation of asemiconducting layer. Those skilled in the art will recognise thatsurface plasmon effects can correspondingly reduce the operationalefficiency of light detecting and photovoltaic devices by trappingincident light, and that the problem may be reduced by the same means asfor emissive devices. The same considerations and remedies apply toother devices, such as electro-optic modulators, switches etc which relyon a dielectric or semiconductor layer provided with at least one metalelectrode.

The inventors of the present invention have analysed the mechanisms bywhich light may be coupled from surface plasmon modes at a metal surfaceinto an emitted mode in the surrounding medium, by means of a periodicmicrostructure. Further the inventors have newly recognised the relativeimportance of surface plasmon modes supported at the inner and outersurfaces of the metal electrode, and the detailed effect of a periodicmicrostructure on the scattering of each. In particular the inventorshave newly recognised that the surface plasmon modes supported at theinner surface of the metal electrode are of dominant importance inreducing the efficiency of devices, and that periodic microstructuresdescribed in the prior art are substantially ineffective in couplinglight from surface plasmon modes at the inner surface of the metalelectrode into emitted modes. Recently Gifford and Hall (App. Phys.Lett. Vol. 80, p 3679-3681, 2002 showed that emission could be enhancedif the periodic microstructure is used to couple the SP modes on theinner and outer metal surfaces together as well as coupling them toemitted radiation. In their work the SP mode at the inner surface isonly coupled out under very specific matching conditions, so providingonly limited recovery of power lost to the SP mode at the inner surface.The inventors of the present invention have further addressed thisproblem by the design and fabrication of a new type of periodicmicrostructure. By the inner surface of the metal electrode, is meantthe surface of the metal electrode which is closer to the emissive layerin a light emitting device eg a dielectric or semiconductor layer. Theinventors have found that the reason for the ineffectiveness of priorart microstructures lies in a destructive interference of light whichoccurs between light produced via i/ and ii/ below:

-   i) Light scattered by a periodic microstructure from the surface    plasmon mode at the inner metal electrode surface i.e. at the    interface between the emissive layer and the metal electrode and    then propagating through the electrode. The periodic microstructure    is substantially present at the inner metal surface.-   ii) Light scattered by a periodic microstructure substantially    present at the outer metal surface by interaction with the    evanescent wave associated with the surface plasmon mode at the    inner metal electrode surface.

According to a first aspect of this invention an optoelectronic devicecomprises:

-   a dielectric layer or a semiconductor layer sandwiched between    electrode structures, wherein at least one of the electrodes is    substantially metal comprising and at least semi-transparent,-   a periodic microstructure in contact with at least one surface of    the substantially metal comprising and at least semi-transparent    electrode,-   characterised in that the structure and positioning of the periodic    microstructure is such that:-   surface plasmon (SP) polariton modes supported mainly at the    interface between the dielectric layer or semiconductor layer and    the metal comprising, semi-transparent electrode-   are substantially scattered into propagating light, said propagation    being out of the plane of the dielectric layer or semiconductor    layer and the metal comprising, semi-transparent electrode    interface.

By contact is meant physical and/or optical contact and by opticalcontact is taken to mean that the electric field associated with the SPmode supported at (or associated with) the dielectric layer orsemiconductor layer and the metal comprising, semi-transparent electrodeinterface has a significant/appreciable amplitude at the periodicmicrostructure.

In practice it will often be the case that the periodic microstructureis in physical contact with the metal comprising semi-transparentelectrode, however there may optionally be further semi-transparent ortransparent layers positioned in between the various layers referred toabove, for example there may be transparent layer(s) in between theperiodic microstructure and the layers either side of the periodicmicrostructure.

Periodic microstructures include grating type structures such as aperiodic sequence of valleys and hills, or a periodic sequence ofgrooves. They also include surfaces that are periodic in more than onedirection on the surface, examples are the simultaneous presence of twograting structures and two dimensionally periodic arrangements of holes,bumps etc.

Periodic microstructures also includes structures which are described asquasi-periodic structures, one example of which is a so called Penrosetiling.

A periodic microstructure in contact with at least one of the electrodesincludes the case where it is the said electrode that ismicrostructured.

By semi-transparent it is meant that said electrode allows sufficientlight through for the device to operate and that the electrode is atleast semi-transparent.

In specific examples and/or in discussions relating to the prior artherein the term emissive layer or light emitting layer is often usedwhich is taken to mean the dielectric or semiconductor layer(s) which is(are) sandwiched between electrodes as referred to in the statement ofinvention. In the case of an electro-optic device which functions as aphotovoltaic device, a photodiode or a photoresistive device, suchdielectric and semiconducting layers may not actually be photoemissivewith a usefully high efficiency but will perform a correspondingelectro-optic function by absorbing light and generating or passingelectrical charge. It will be understood by those skilled in the fieldthat the operation of such devices and others can be understood byconsidering light absorption rather than emission and charge separationrather than recombination and applying these changes mutatis mutandisaccording to the device under discussion.

An interface with the air is taken to mean an interface with the outeredge of the structure. A further layer may be added (encapsulation) atthis interface—typically by using a further transparent layer.

Preferably the substantially metal comprising electrode means that saidelectrode comprises a metallic layer, more preferably the electrode is ametallic electrode such as aluminium. Preferably the metal comprisingelectrode is the cathode. Preferably the cathode is made from aluminium.

It was stated earlier in the current application that the presentinventors have found that the reason for the ineffectiveness of priorart microstructures lies in a destructive interference of light whichoccurs between light produced substantially via two routes. The currentinvention in effect results in a substantial lack of such destructiveinterference.

Preferably the substantial lack of destructive interference is taken tomean that the interference of the light arising from each of the aboveidentified scattering routes from a surface plasmon is such that at achosen wavelength at least 50% of the power which in a planar structurewould be trapped as a surface plasmon, is emitted as useful radiation.Preferably such substantial lack of destructive interference will leadto an overall increase in the external efficiency of the device of atleast 10%.

Being out of the plane of the dielectric layer or semiconductor layermeans that the propagating light is suffciently out of the plane so thatit can be emitted from the device as useful light.

SP modes supported mainly at the interface between the said layers andelectrode is taken to mean the interface mode that comprises anoscillating electromagnetic field associated with an oscillating surfacecharge distribution in the metal, the electromagnetic field being suchthat it decays exponentially with distance away from the interface.

Preferably the periodic microstructure is selected from the followingstructures:

-   -   the metal comprising electrode comprises a grating type        structure on each of its surfaces (i.e. possessing wavelength        scale periodic microstructure that is periodic in at least one        direction in the plane of the device), wherein the relationship        between the microstructure of the two metal comprising surfaces        is such that they are out of phase by π radians or substantially        π radians.    -   a grating type structure present only at the interface between        the metal comprising electrode and the layer into which        electrons are injected ie the semiconductor or dielectric layer.    -   a grating type structure present at the metal comprising        electrode/air (or metal comprising electrode/encapsulation)        interface only.    -   a further dielectric layer present at the surface of the metal        comprising electrode remote from the dielectric/semiconductor        layer, on which is present a grating type structure.

In addition all of these structures may be such that they possesswavelength scale periodic microstructure in more than one direction inthe plane of the device so that the microstructure extends in both the xand y directions in the plane of the device, as noted below.

For those periodic microstructures listed above the grating typestructure may be a series of holes preferably sub-wavelength which havebeen made in the metal comprising electrode. Alternatively it may be aseries of lines etched in to the appropriate surface, e.g. by laser. Itmay also be a series of bumps and dimples. Such techniques are wellknown to those skilled in the art.

By wavelength scale periodic microstructure is taken preferably to meana periodic arrangement of bumps and hollows or grooves and ridgeswherein the distance between successive bumps or grooves and the like isof the order of the wavelength involved or less.

By substantially π radians is preferably taken to mean between andincluding π/2 and 3π/2 radians.

To optimise the light extracted the periodic microstructure may beperiodic in more than one dimension. For example a second (andoptionally a third) microstructure or corrugation may be added, thusallowing surface plasmons propagating in all in-plane directions to becoupled to radiation. It has been demonstrated that this can be anefficient process, Worthing et al., Appl. Phys. Lett., 79, 3035 (2001).Multi-periodic microstructures include gratings etched at substantially90° to each other where two gratings are used or at substantially 60° toeach other where three gratings are used. It is understood that periodicmicrostructures may be fabricated by a variety of known methodsincluding but not restricted to photolithography, e-beam lithography,chemical or plasma etching, laser machining or ablation, mechanicalscribing or ruling, embossing, and selective exposure of a photopolymer.

By careful design of the periodic microstructure surface plasmon modesare recovered as useful radiation which in the case of a device such asan LED results in increased efficiency and increased brightness.Similarly it allows for sensors which may work more efficiently in lowerlighting conditions

Preferred optoelectronic devices include photodiodes, photovoltaics,light emitting diodes both organic and inorganic, light emitting polymerdevices, emissive displays and solid state lighting elements.

The dielectric or semiconductor layer may itself consist of one or morelayers. For example in the case of an LED the dielectric orsemiconductor layer typically possesses the following three properties:electron transporting (ET); hole transporting (HT); light emitting (LE).If the layer of material is a single layer then the single layer ofmaterial must exhibit all three properties. For the case when the layerof material is a single layer then the material may consist of a singlematerial, for example in a typical organic LED polyphenylenevinylene, orby mixing two or more materials with appropriate properties together,for example N,N′-diphenyl-N,N′-ditolylbenzidine (HT), Coumarin 6 Laserdye (LE) and t-Butylphenyl 4-biphenylyl-oxadiazole (ET) which may beabbreviated to PBT. For the case when the layer of organic materialcomprises more than one layer then suitable examples include:

-   i/Layer 1=HT layer, Layer 2=LE layer, Layer 3=ET layer-   ii/Layer 1=HT layer, Layer 2=material which acts as an ET medium but    also emits light (LE), for example Aluminium tris    8-hydroxyquinolinate (Alq3)-   iii/Layer 1=HT and LE, Layer 2=ET-   iv/The LE material may be doped in small quantities—typically 0.5%    into ET or HT or both. Typical doping agents are coumarin 6 or    pentaphenyl cyclopentadiene. In cases where concentration quenching    is not severe, larger doping concentrations, for example, 10% may be    used. Examples of suitable dopants include rubrene, and complexes of    terbium, europium and iridium.

Preferably in the case where the layer of material is a multiplicity oflayers, then the layer adjacent to the cathode preferentially transportselectrons and/or the layer adjacent the anode preferentially transportsholes. Preferably the luminescent material has a high quantum efficiencyof luminescence. The luminescent component may be combined with a chargetransporting material or may be present in a separate layer.

The dielectric or semiconductor layer may be deposited on the anode byany of the following techniques: thermal evaporation under vacuum,sputtering, chemical vapour deposition, spin depositing from solution orother conventional thin film technology. Other suitable techniques willbe apparent to those skilled in the art.

The thickness of the dielectric or semiconductor layer is typically30-2000 nm, preferably 50-500 nm. The device may contain further layerswhich are situated next to the electrodes and the semiconductor ordielectric layer—these further layers may be conducting or insulatingand act as a barrier to diffusion of the electrode material or as abarrier to chemical reaction at the electrode and dielectric orsemiconductor layer interface and/or may act to facilitate injection ofcharge into the adjacent layer. Examples of suitable materials for thesefurther layers include emeraldine which prevents indium diffusion intothe layer of dielectric or semiconductor from an ITO electrode, or, forthe same reason, copper phthalocyanine may be used; alternatively theaddition of a thin layer (˜0.5 nm) of lithium or magnesium fluoride atthe interface between a lithium electrode and the dielectric orsemiconductor layer may be used.

Hence preferably according to the present invention an optoelectronicdevice comprises:

-   a dielectric layer or a semiconductor layer sandwiched between    electrode structures, wherein at least one of the electrodes is    metallic and semi-transparent;-   a periodic microstructure in contact with the metallic    semi-transparent electrode-   wherein the periodic microstructure is given by one of the following    structures:    -   a grating type structure which is in contact with both sides of        the metallic semi-transparent electrode, wherein the        relationship between the microstructure of the two surfaces of        the grating type structure is such that they are out of phase by        π or substantially π;    -   a grating type structure present only at the interface between        the metallic semi-transparent electrode and the semiconductor or        dielectric layer;    -   a grating type structure present at the metallic        semi-transparent electrode/air interface only;    -   a grating type structure present on a further dielectric layer        present at the surface of the metallic semi-transparent        electrode said surface being remote from the dielectric layer or        semiconductor layer.

Preferably the metallic semi-transparent electrode is the cathode.

For all of the aspects of the present invention at any of the interfacesof the optoelectronic device there may be present transparentlayers—when such a layer is used at the interface with the air then thisis generally referred to as an encapsulation layer. For example thesemi-transparent electrode air interface may have an encapsulationlayer.

All of the previously disclosed statements in relation to the firstaspect of the invention are applicable to this preferred aspect of theinvention.

The invention will now be described by way of example only withreference to the following Figures:

FIG. 1 illustrates a typical planar LED structure

FIG. 1 a illustrates a light emitting structure such as that illustratedin FIG. 1 with wavelength scale periodic corrugation incorporated

FIG. 2 illustrates a power dissipation spectrum for an emitter embeddedin an emissive layer (Alq₃) in close proximity (30 nm) to a thin (30 nm)silver layer (simulating the cathode)

FIG. 3 illustrates emission spectra recorded at a polar emission angleof 19 degrees. The mode assignments are made by reference tomeasurements and calculations of the dispersion of the optical modes ofthis structure. The spectra were obtained by using a device asillustrated in FIG. 1 a

FIGS. 4 a-d illustrate periodic microstructures for use in the presentinvention

-   -   4 a illustrates a bi-grating where the individual gratings are        out of phase by π with respect to each other    -   4 b illustrates a periodic microstructure at the cathode/air        interface only    -   4 c illustrates a periodic microstructure at the interface        between cathode and dielectric or semiconductor layer only    -   4 d illustrates the case where a further dielectric layer is        deposited on the cathode and wherein there is a periodic        microstructure at the further dielectric layer/air interface        only

FIG. 5 illustrates a device according to the present invention

FIG. 6 illustrates emission spectra (photoluminescence) recorded at apolar emission angle of 11 degrees by use of the device in FIG. 5

FIGS. 7 a-c illustrate a device according to the present invention

FIG. 8 illustrates a multiplexed addressed device according to thepresent invention.

In order that the invention may be more readily understood it isnecessary to more fully understand some of those processes occurring inoptoelectronic devices such as LEDs.

To gain a quantitative idea of the nature of the problem that SP modesrepresent, the current inventors have calculated the power coupled tothe different modes using well established techniques of Ford et al,Phys. Rep., 113, 195 (1984) and Wasey et al, J. Mod. Opt., 47, 725(2000). FIG. 2 illustrates the power coupled from an emitter as afunction of in-plane wavevector. By way of example the structureinvestigated comprises a 60 nm layer of light emitting material (e.g.Alq₃) fabricated on top of a glass substrate, and coated with 30 nm ofsilver, to simulate a cathode. The calculation is for an emitter in themiddle of the emissive layer, i.e. 30 nm from the metal surface. Thedifferent peaks in FIG. 2 represent different modes to which the emittermay couple; the area under each peak represents the power coupled tothat mode. In addition to radiation a very significant amount of poweris coupled to surface plasmon modes, notably the SP mode associated withthe metal/emissive material interface. (There are two SP modes in thisstructure, one associated with the metal/air interface, the otherassociated with the metal/emissive material interface). If nothing isdone to recover trapped guided modes such as these SP modes then theyrepresent a wasteful decay route for the excitons and will reduce theefficiency of the device.

From FIG. 2 it is clear that molecules lose a very significant fractionof their energy to the SP mode associated with the metal/emissivematerial interface (in this case the Ag/Alq₃ interface). If nothing isdone to recover the power coupled to this SP mode then the efficiency ofthe device will always be sub optimal.

The structure in FIG. 1 a was fabricated as follows, in this instancewith the absence of an anode. A film of Shipley Megaposit 700 wasdeposited onto a planar silica glass slide 1 by spin coating. Thecorrugated surface, taking the form of a diffraction grating (λ_(g)=412nm) was then produced in the photoresist by means of holographiclithography Kitson et al, IEEE. Phot. Tech. Lett., 8, 1662, 1996. Thestructure was then placed in a reactive ion etcher where CHF₃ and O₂gases were used to etch the grating profile into the silica substrateand to remove the remainder of the photoresist layer. Onto the texturedsubstrate the Alq₃ (70 nm) layer 3 and silver (30 nm) films 4 weresequentially evaporated. The pitch of the grating needs to beappropriately chosen. The grating produced here had a surface profilethat can be approximated by a sine wave, y(x)=a₀ sin(k_(g)x) wherek_(g)=2π/λ_(g), λ_(g) is the pitch of the grating. In the present casethe pitch was 412 nm and the amplitude of the surface profile was a₀˜25nm. The condition on the pitch is that the surface plasmon modessupported by the structure (see FIG. 2) can be Bragg scattered by theperiodic corrugation and in so doing be coupled to far-field radiation.Thus the pitch of the grating must be such that,k _(sp) ±nk _(g) <±k ₀where k₀ is the wavevector of a photon at the emission frequency in freespace, k_(SP) is the in-plane wavevector of the SP mode that is to becoupled to radiation and k_(g) is the Bragg vector associated with thecorrugation. The integer n is the order of the Bragg scattering andusually takes the value n=1, i.e. lowest order scattering. The amplitudeis an important parameter in determining the efficiengy of thescattering process, and should typically be of the order of ≈λ₀/10 whereλ₀ is the free space emission wavelength of the emissive material.

The emission spectrum for light emerging through the metallic layershould include features due to the SP modes associated with thestructure. The results, shown in FIG. 3, show that emission associatedwith the metal/air plasmon dominates the recorded emission. Given thatthe emitters couple much more strongly to the surface plasmon associatedwith the metal/emissive layer interface (see FIG. 2), these resultsindicate that the metal/emissive layer surface plasmon is very poorlycoupled to radiation by this structure.

The inventors of the current invention have found that the reason forthis weak coupling of the metal/emissive layer SP mode to radiation, isa result of destructive interference between the two scatteringprocesses that may couple the SP mode to radiation. The two processesare:

-   1. The SP mode is scattered by the lower (metal/emissive layer)    corrugation and propagates through the metal to emerge in the air.-   2. The SP mode penetrates through the metal, is scattered by the    upper (metal/air) corrugation, to emerge in the air.

The net phase incurred in these two scattering processes differ by π sothat they interfere destructively. By contrast, the upper, metal/airsurface plasmon has only one route for scattering so as to produceradiation in the air—there can thus be no cancellation due todestructive interference, thus explaining why this mode is seen in thespectra shown in FIG. 2, despite being only very weakly excited by theemissive layer.

FIGS. 4 a-d illustrate examples according to the present invention ofhow the phase difference between SP modes may be altered and theinterference conditions changed from destructive to constructive.

In FIG. 4 a the two periodic microstructures or surface corrugations areslipped by π relative to each other.

In FIG. 4 b there is present a periodic microstructure at thecathode/air interface only.

In FIG. 4 c there is present a periodic microstructure on the dielectricor semiconductor layer (may be generally referred to as an emitter oremissive layer) and the interface with the cathode only.

In FIG. 4 d there is a layer of dielectric or semiconductor (the anodeis not shown), a metal comprising cathode which is flat or substantiallyflat on both surfaces. Instead of the next layer being air there isdeposited a further dielectric layer with a periodic microstructure.

With respect to the structure illustrated in FIG. 4 d this affordsadvantages in terms of the fabrication process in that the finaldielectric layer could be deposited on an already formed planar metalcomprising cathode.

FIG. 5 illustrates a device according to the present invention which isused to illustrate the effect of use of a periodic microstructureaccording to the present invention. There is present an anode, a layerof dielectric or semiconductor, a metal comprising cathode and insteadof the next layer being air there is deposited a further dielectriclayer 31 possessing periodic microstructure on top of the cathode 4.

A structure similar to that shown in FIG. 5 was fabricated; but withoutthe presence of an anode. A layer of emissive material (Alq₃) wasdeposited by vacuum sublimation, after which a thin (˜30 nm) layer ofmetal (Silver) was deposited to represent a cathode. A thin (˜100 nm)layer of photoresist was then added by spin coating. The photoresist wasthen exposed to an interference pattern produced by interfering twolaser beams, chemical development being used to translate the exposuremodulation into a physical modulation of the photoresist.

The emission spectrum from the structure illustrated in FIG. 5 is shownin FIG. 6. The SP mode associated with the metal comprisingelectrode/dielectric or semiconductor layer (generally referred to asthe emissive layer) interface is now the stronger of the surface plasmonfeatures (when compared to the intrinsic emission spectrum).

FIGS. 7 a-c illustrate further examples of LED devices constructedaccording to the present invention. A substrate typically made of glass1 is sequentially overlaid with an anode 2 typically made of Indium TinOxide (ITO), a dielectric or semiconductor layer 3 (often referred to asany of an emissive layer or a light emitting layer) which may beessentially of an organic or inorganic nature. This layer may itselfconsist of one or more layers represented in FIG. 7 c as 3 a, 3 b, 3 c.The layer of material 3 possesses the following three properties:electron transporting (ET); hole transporting (HT); light emitting (LE).If the layer of material 3 is a single layer then the single layer ofmaterial 3 must exhibit all three properties. For the case when thelayer of material 3 is a single layer then the material may consist of asingle material, for example in an Organic LED (OLED)polyphenylenevinylene, or by mixing two or more materials withappropriate properties together, for exampleN,N′-diphenyl-N,N′-ditolylbenzidine (HT), Coumarin 6 Laser dye (LE) andt-Butylphenyl 4-biphenylyl-oxadiazole (ET) which may be abbreviated toPBT. For the case when the layer of material 3 comprises more than onelayer then suitable examples include:

-   i/3 a=HT layer, 3 b=LE layer, 3 c=ET layer-   ii/3 a=HT layer, 3 b=material which acts as an ET medium but also    emits light (LE), for example Aluminium tris 8-hydroxyquinolinate    (Alq3)-   iii/3 a=HT and LE, 3 b=ET-   iv/The LE material may be doped in small quantities—typically 0.5%    into ET or HT or both. Typical doping agents are coumarin 6 or    pentaphenyl cyclopentadiene.

Preferably in the case where the layer of material 3 is a multiplicityof layers, then the layer adjacent to the cathode preferentiallytransports electrons and/or the layer adjacent the anode preferentiallytransports holes. Preferably the luminescent material has a high quantumefficiency of luminescence. The luminescent component may be combinedwith a charge transporting material or may be present in a separatelayer.

The layer of material 3 may be deposited on the anode 2 by any of thefollowing techniques: thermal evaporation under vacuum, sputtering,chemical vapour deposition, spin depositing from solution or otherconventional thin film technology.

The thickness of the layer of material 3 is typically 30-2000 nm,preferably 50-500 nm. The device may contain layers 21 a and 21 b (seeFIG. 7 c) which are situated next to the electrodes 2 and 4, theselayers 21 a and 21 b may be conducting or insulating and act as abarrier to diffusion of the electrode material or as a barrier tochemical reaction at the electrodes 2,4 and layer of organic material 3interface. Examples of suitable materials for 21 a and 21 b includeemeraldine which prevents indium diffusion into the layer of (organic)material 3 from an ITO electrode, or, for the same reason, copperphthalocyanine may be used; alternatively or additionally the additionof a thin layer (˜0.5 nm) of lithium or magnesium fluoride at theinterface between a lithium electrode and the layer of (organic)material 3 may be used.

The metal comprising periodically microstructured cathode illustrated inFIG. 7 a is typically, when compared to standard cathodes for use inLEDs, thinner. The cathode possesses a planar lower surface and asinusoidally modulated top surface, the pitch of the modulation is ofthe order of λ₀/2, for emission in the visible the pitch will thus be˜250-300 nm. The mean thickness of the cathode may be ˜20-40 nm, whilstthe amplitude of modulation is typically ˜10-30 nm. The textured profileof the top metal comprising layer is produced by a number of techniquesincluding etching through a photoresist grating formed on top of thecathode. Alternatively a cathode in which the modulation is deep enoughto penetrate the cathode completely, leaving a metal film full of holesmay be provided. In another variant the semiconductor or dielectriclayer may have its top surface modulated and a cathode added that has aplanar top surface.

Another variant device structure has a textured dielectric layer 22added above the cathode, as shown in FIG. 7 b (see layer 31 in FIG. 5).Here the cathode is relatively thin and planar, ˜30 nm. It is covered bya textured dielectric layer. The textured dielectric layer serves thesame purpose as the directly textured cathode of FIG. 7 a, it allows thesurface plasmon modes to be Bragg scattered out of the device as usefulradiation. The thickness of this layer must be thin enough to avoid theintroduction of further waveguide modes. Typically this may be avoidedif the textured dielectric layer has a thickness of the order of onesixth the emitted wavelength if the dielectric layer has opticalproperties typical of common organic materials. It is further desirablefor the texture to comprise a thickness modulation of at least one tenththe emitted wavelength in order to achieve strong scattering.

For those devices described by FIGS. 7 a-c the periodic microstructuresof FIGS. 4 a and 4 c may alternatively be incorporated in to devices 7a-c.

The device of FIG. 7 a-c may be a single pixel device or it may bematrix addressed. An example of a matrix addressed OLED is shown in planview in FIG. 8. The display of FIG. 8 has the internal structuredescribed in FIG. 7 a-c but the substrate electrode 5 is split intostrip-like rows 5 l to 5 m and similar column electrodes 6 l to 6 n,this forms an m×n matrix of addressable elements or pixels. Each pixelis formed by the intersection of a row and column electrode.

A row driver 7 supplies voltage to each row electrode 5. Similarly, acolumn driver 8 supplies voltage to each column electrode. Control ofapplied voltages is from a control logic 9 which receives power from avoltage source 10 and timing from a clock 11.

An organic photovoltaic device comprises a thin stratified layer oforganic semiconductor placed between two electrically conductingelectrodes, at least one of which is at least semi-transparent. Thestratified layer of organic semiconductor is so disposed that theorganic semiconductor in contact with one electrode is a n-typesemiconductor, and that in contact with the other electrode is a p-typesemiconductor. Within the thickness of the organic layer, a p-n junctionis formed. Light absorbed in the organic semiconductor layer causeselectronic excitation of one or more molecules to provide an excitedstate molecule, excimer or exciplex which may dissociate to provide apair of charge carriers of opposite charge. If the pair of chargecarriers is formed in close proximity to the organic p-n junction, onecharge carrier may diffuse across the junction resulting in permanentseparation of the charges. Further diffusion of the charge carriers tothe electrodes results, under constant illumination, in an electricalpotential difference and/or external current flow between theelectrodes. In order to achieve high conversion efficiency of lightenergy into electrical power, light should preferably be absorbed by theorganic semiconductor layer very close to the p-n junction in order toincrease the probability of diffusion of a charge carrier across thejunction. Means to increase this probability include creation of aroughened or diffuse p-n junction, inclusion of the organicsemiconductor layer in an optical resonant cavity having an antinodeclose to the plane of the p-n junction, and having light traverse theorganic semiconductor layer at an oblique angle. It will readily beunderstood by those skilled in the art that according to the physicalprinciple of reversibility of light the effects described above relatingto an organic light emitting device and the effects of a metal electrodethereon are exactly paralleled in a photovoltaic device on which lightis incident. Means by which the efficiency of out-coupling of light froman organic LED are improved, also increase the efficiency of in-couplingof light into an organic photovoltaic device and may be applied toincrease its efficiency and power generation capacity.

An organic photodiode comprises a similar structure to that of anorganic photovoltaic device but is used as a device to detect or measureincident light. Such a photodiode is often operated under an externallyapplied electrical potential difference, and the resulting photocurrentis monitored. It will be apparent that like organic photovoltaicdevices, organic photodiodes may be improved by increasing theefficiency of in-coupling of externally incident light.

1. An optoelectronic device comprising: a dielectric layer or asemiconductor layer sandwiched between electrode structures, wherein atleast one of the electrodes is substantially metal comprising and atleast semi-transparent, a periodic microstructure in contact with atleast one surface of the substantially metal comprising and at leastsemi-transparent electrode, characterised in that the structure andpositioning of the periodic microstructure is such that: surface plasmon(SP) polariton modes supported mainly at the interface between thedielectric layer or semiconductor layer and the metal comprising,semi-transparent electrode are substantially scattered into propagatinglight, said propagation being out of the plane of the dielectric layeror semiconductor layer and the metal comprising, semi-transparentelectrode interface.
 2. A device according to claim 1 wherein theperiodic microstructure is selected from the following structures themetal comprising electrode comprises a grating type structure on each ofits surfaces, wherein the relationship between the microstructure of thetwo metal comprising surfaces is such that they are out of phase by πradians or substantially π radians; a grating type structure presentonly at the interface between the metal comprising electrode and thesemiconductor or dielectric layer; a grating type structure present atthe metal comprising electrode/air interface only; a further dielectriclayer present at the surface of the metal comprising electrode remotefrom the dielectric/semiconductor layer, on which is present a gratingtype structure.
 3. A device according to claim 2 wherein the periodicmicrostructure is selected from a grating type structure present at themetal comprising electrode/air interface only wherein there is presentan encapsulating layer on the electrode.
 4. A device according to claim1 wherein the periodic microstructures are a periodic sequence ofvalleys and hills, or a periodic sequence of grooves.
 5. A deviceaccording to claim 1 wherein the periodic microstructures are a gratingtype structure which is a series of holes in the metal comprisingelectrode.
 6. A device according to claim 1 wherein the periodicmicrostructures are periodic in more than one direction on the surface.7. A device according to claim 1 wherein the periodic microstructuresare sub-wavelength.
 8. A device according to claim 1 wherein the metalcomprising electrode is an aluminium cathode.
 9. A device according toclaim 1 wherein the device is chosen from a light emitting diode, aphotovoltaic cell or a photodiode.
 10. A device according to claim 9wherein the light emitting diode is an organic light emitting diode.